Several reasons convinced me to move cintoo Messages from jMock to EasyMock 2. The main reason is that jMock has no static importable methods for mock creation and verification. The way to do this in jMock is to extend a jMock TestCase class. Unfortunately this class depends on JUnit, so when moving to TestNG I still had to keep the JUnit jar around. Also TestNG promotes a no-inheritance test structure where jMock just doesn't fit in. The second reason is that I was bitten by method name refactorings several times. As some methods were equal in different classes, IDEA wrongfully replaced the methods strings in jMock which broke lots of tests. Another minor reason is that jMock doesn't seem to move very fast.
The transition has been made easier by EasyMock as that framework adopted a lot of ideas from jMock in version 2.x (I think :-). I still hope to use the jMock spinoff assertion DSL though with assertThat( eq(5), ...).
In the last years since the first jMock release I've been convincing clients to use jMock for their unit testing. I still think jMock is great for JUnit, but it just doesn't work with TestNG in the current version.